The present invention relates to an apparatus for detecting an image area of a thin plate, wherein a surface of the thin plate is photographed by photographic means, and picture areas of respective zones of the plate are detected to control an amount of ink to be applied thereto
FIG. 1 is a side view showing the schematic configuration of a conventional offset printing press. Ink and damping water are supplied from an inking unit 3 and a damping unit 4 to the surface of a plate 2 wound around a plate cylinder 1, so that an image is formed on the image portion of the plate 2. The image is transferred to a blanket cylinder 5 brought into light contact with the plate cylinder 1. The image is then transferred to a sheet 8 passing through the blanket cylinder 5 and an impression cylinder 7. In this case, ink flowing out from an ink duct 9 is transferred through nips which have different phases and which are formed between the rollers such as distributing rollers 10 and vibrating rollers 11 which have different diameters. Therefore, the ink can be uniformly distributed onto the surface of the plate 2 along the circumferential direction thereof. The ink consumed in the image portion of the plate 2 can be sequentially replenished along an ink path. When the amount of ink supplied to the plate 2 is smaller than a predetermined amount, the printing density becomes excessively low. However, when the amount of ink becomes larger than the predetermined amount, the printing density becomes excessively high, resulting in degradation of the printing quality. In addition, when the ink is kept unused in the inking rollers, the glossy effect of the printed surface is lost. Proper control of the amount of ink is one of the most important factors in printing.
For this purpose, a conventional automatic ink adjusting apparatus has been proposed, wherein the ink duct 9 is divided into 30 to 50 zones along the width of the sheet 8, and ink duct keys 12 are provided for the respective zones, and wherein a compact motor 13 with a potentiometer is driven in response to an instruction from a control unit to adjust a proper amount of ink for each zone. However, the distribution of the image areas of the respective zones varies in accordance with the type of printed matter. When ink adjustment for the respective zones is performed after printing is started, a great amount of spoilage is generated during the adjustment period. Such adjustment becomes a time-consuming operation. Therefore, it is preferred that the picture areas of the respective zones of the printing plate are calculated before printing is started, and that coarse adjustment of the ink duct 9 by the keys 12 is then performed or an amount of ink is determined in accordance with a print sheet number prior to printing.
Various types of conventional picture area measuring apparatus for plates have been proposed. According to a typical measuring apparatus, a plate 2 is mounted on a plate cylinder 1 or another high-speed cylinder, and the cylinder is rotated. A detector is slid along the axial direction of the cylinder, and detected values are accumulated in units of zones. However, when the plate 2 is mounted on the plate cylinder 1, a space for scanning the plate along the axial direction is required, thus degrading the operability of the normal printing operation. In addition, as the detector is used on the printing press subjected to great vibrations, high precision cannot be guaranteed. In another conventional measuring apparatus using a high-speed rotary cylinder only for measurement, as the plate 2 must be mounted on the rotary cylinder in the same manner as in the case wherein the plate 2 is mounted on the plate cylinder 1, the operation becomes cumbersome. When a thin aluminum plate for an offset printing press is used, the mounting holes may be deformed, thus degrading the printing precision.
Another conventional measuring apparatus is also known. According to this apparatus, a plate 2 is horizontally placed on a table. A detector having a slit for covering the zone direction is slid along the plate surface. The detected values are accumulated in units of zones. In this apparatus, a detector scanning mechanism is complicated, and so the manufacturing cost is high. An elongated slit-like detector cannot be obtained. Instead, a plurality of photoelectric elements must be linearly aligned. In this sensor, it is difficult to achieve uniform element characteristics. As a result, high detection precision cannot be obtained.
Another conventional measuring apparatus using a surface light-emitting element and a surface detecting element is known. In this apparatus, an original film is clamped between a surface light-emitting element and a surface detecting element to measure the amount of light transmitted through the film. When a plurality of identical images are aligned within a single plate, the measured results are obtained in units of zones in accordance with the layout of the image, resulting in high cost and a cumbersome operation. In addition, it is very difficult to prepare a large-size printing press of suitable precision for commercial use.
Still another conventional measuring apparatus is proposed wherein image signal components are accumulated in units of zones when the image signal is transmitted as a facsimile signal. This apparatus is an expensive system using a complicated program and is used only in the newspaper industry or the like, and cannot therefore be regarded as a widespread standard apparatus.
In an attempt to solve all the conventional problems, still another conventional apparatus is provided, wherein a plate is placed on a horizontal table and is photographed by a photographic means while the plate is illuminated with sunlight or by an illuminating unit, and the resultant image signal is converted to an electrical signal to obtain correction values of the ink duct keys 12. However, in this case, when the plate is exposed to light from a lateral direction while the plate horizontally placed on the table is exposed under the illumination light so as to perform image area detection, the following problem occurs.
When the plate 2 is placed on the horizontal table and is illuminated from above, the plate is greatly influenced by external light, thereby degrading the measuring precision. In addition to this disadvantage, when the plate is illuminated from the side at which the plate 2 is mounted, the working space of the operator interferes with the illumination unit, and thus a sufficient intensity of light cannot be obtained and the mounting efficiency of the plate is degraded. However, in order to prevent this interference, when the plate 2 is illuminated from only the direction perpendicular to the mounting direction of the plate 2, the light efficiency is degraded since the distance from the center of the plate 2 is increased. As a result, irregular illumination occurs, thus disabling maintenance of high measuring precision.